In 1783, seeing his wife stricken by serious illness, Mozart is said to have made a promise to God: in case Constance Weber recovered, he would compose a great mass as a token of his gratitude. Constance recovered and Mozart started creating a monumental work which, for reasons unknown, he didnít finish. Though not completed, the Great Mass in C minor was premiered in St. Peterís Abbey in Salzburg, with Constanze singing the soprano parts. To its full right, it is held to be one of the masterpieces of the Salzburg composer.